This invention relates to the manufacture of cathode ray tubes (CRTs), and more particularly relates to the high voltage processing of CRT mounts.
In the manufacture of CRTs for color television, it is necessary to process the electron gun assembly (also called the "mount") after it has been sealed into the neck of the CRT, in order to minimize the occurrence of internal arcs during later CRT operation. Modern color CRTs are particularly susceptible to such internal arcing due to their relatively high operating voltages (e.g., 25 KV and higher), and complex electron gun structures having relatively small interelectrode spacings (e.g., mils). In high voltage processing (also called conditioning or spot knocking) internal arcing between electrodes is purposely induced to remove microscopic sources of field emission such as foreign particles and interelectrode projections, which could otherwise lead to detrimental arcing during later tube operation. To be effective, such conditioning should induce arcing not only in the upper gap (gap between the final focusing electrode and final accelerating electrode), but also in the lower gap (gap between the focusing electrode and the final grid electrode).
In U.S. Pat. No. 3,736,038, arcing in the upper gap is achieved by grounding the electrodes and impressing a high voltage above the operating voltage across the accelerating electrode (4) and ground. In addition, a resistor is placed between the focusing electrode (6) and ground, thereby causing arcing in the lower gap as well.
In U.S. Pat. No. 4,214,798, arcing in the lower gap is achieved by allowing the focusing electrode (G.sub.3) to "float", that is, be unconnected, and by impressing a low frequency pulsed voltage across the final accelerating electrode (anode) and the other interconnected electrodes. Optionally, a second high frequency pulsed voltage is also impressed across these electrodes, said to increase the effectiveness of the spot knocking procedure.
Floating the G.sub.3 electrode is said to have the advantages of eliminating the need for separate low voltage supplies as well as the need for providing socket leads for the focusing electrode. The use of pulsed conditioning voltages is said to have the advantage of enabling higher voltages without suffering adverse effects such as neck crazing and electrode metal sputtering.
The low frequency pulsed voltage is applied to the anode via the anode button, a metal contact extending through the CRT glass funnel sidewall. The anode and anode button are interconnected by an internal conductive coating on the funnel sidewall and upper portion of the neck, as well as by metal snubbers extending from the anode to the internal coating.
Because the low frequency pulsed voltage is a half wave rectified AC voltage with the positive portion clamped to ground, the anode voltage is negative, and the internal coating and snubbers are also negative with respect to the adjacent floating focusing electrode. This condition (negative voltage) has been found to enable field emissions from the snubbers and coating to occur, which can result in undesirable crazing or even cracking of the neck glass.
In addition, when known high voltage conditioning methods are practiced, in particular on the new mini-neck color CRTs, arcing at undesired locations sometimes occurs both externally between base pins, and internally between cathodes and heaters.
Accordingly, one object of the invention is to effectively high voltage condition the upper and lower gaps of an electron gun mount without inducing undesirable neck crazing and electrode sputtering.
Another object of the invention is to effectively high voltage condition mounts without inducing arcing at undesired locations in these mounts.
A further object of the invention is to high voltage condition CRT mounts in a manner to minimize or substantially eliminate external arcing between the base pins.